This invention generally relates to electronic registration that is implemented by applying image processing to the image data and, more particularly, to a system to improve image registration in a binary image path by electronic compensation of scan line output through a raster output scanner, array exposure device (more generally, array imaging systems) or other like image output terminals.
Electrophotographic marking is a well-known and commonly used method of copying or printing documents. In general, electrophotographic marking employs a charge-retentive, photosensitive surface, known as a photoreceptor, that is initially charged uniformly. In an exposure step, a light image representation of a desired output focused on the photoreceptor discharges specific areas of the surface to create a latent image. In a development step, toner particles are applied to the latent image, forming a toner or developed image. This developed image on the photoreceptor is then transferred to a print sheet on which the desired print or copy is fixed.
The electrophotographic marking process outlined above can be used to produce color as well as black and white (monochrome) images. Generally, color images are produced by repeating the electrophotographic marking process to print two or more different image layers or color image separations in superimposed registration on a single print sheet. This process may be accomplished by using a single exposure device, e.g., a raster output scanner (ROS) or image bar using array optics, where each subsequent image layer is formed on a subsequent pass of the photoreceptor (multiple pass) or by employing multiple exposure devices, each writing a different image layer, during a single revolution of the photoreceptor (single pass) or by employing multiple exposure devices, each writing a different layer on different photoreceptors (often referred to as a tandem architecture). While multiple pass systems require less hardware and are generally easier to implement than single pass systems, single pass systems provide much greater print speeds.
In generating color images, the ability to achieve precise registration of the image layers is necessary to obtain printed image structures that are free of undesirable color fringes and other registration errors. Precise registration of image layers in a single pass machine requires precise registration from one output exposure device, either ROS or array, to the next. One major cause of misregistration in multiple output exposure device systems is the differences in the trajectory and placement of the exposing spots by each exposure device in the imaging system. The following discussion will focus primarily on electrophotographic printing systems using ROS exposure sub-systems, recognizing that any digital printing system based upon multiple color separations will induce similar misregistration errors in output.
In general, a conventional ROS repeatedly scans a data modulated light beam over a photoreceptor surface in accordance with a predetermined raster scanning pattern to generate an image. Typically, a conventional ROS includes a laser diode or similar device to generate a light beam that is modulated in response to received data. The ROS further includes a rotating polygonal mirror block to repeatedly scan the light beam across the photoreceptor. As the photoreceptor is advanced in the process direction, the ROS repeatedly scans the modulated light beam across the surface of the photoreceptor in a fastscan direction that is orthogonal to the process direction. Array based printing systems, on the other hand, typically employ a plurality of imaging sources to illuminate desired portions of the photoreceptor or to apply ink to a substrate whole rows at a time.
To continue the present example, each scan of a ROS beam across the photoreceptor (generally identified for all writing systems herein as a beam scan or simply as a scan) ideally traces a straight line across the surface of the photoreceptor in the process direction. However, in ROS systems, variations in the optics introduce pixel positioning errors from distortions in the trajectory of each beam scan. Typically, each ROS introduces different and independent pixel positioning errors. Thus, in a machine with more than one ROS, each ROS will likely have a different beam scan trajectory. Indeed, there are a multitude of sources of trajectory error, which include pyramidal errors in the polygonal mirror, skew in the alignment of the ROS to the photoreceptor, and the optical aberration known as geometric distortion. Errors can be in either the fastscan or slowscan direction. In an array-based printing system, there are also a multitude of sources of pixel location error, only some of which coincide with those in a ROS. In sum, systems employing multiple color separations presently suffer from image artifacts introduced by pixels of one exposure device being mis-registered from pixels of another exposure device.
To achieve the registration necessary to generate color images that are free of undesirable registration errors, the pixel positioning of each ROS is preferably within xc2x15 microns of the pixel positioning of every other ROS. More preferably, pixels from each ROS should register precisely with pixels from every other ROS. Such tight registration tolerances are very difficult and very expensive to achieve solely by opto-mechanical means. Systems for compensation and/or correction of beam scan distortions to improve registration errors have been proposed. However, many of these proposed systems correct only one type of distortion and often are themselves complex and expensive to implement.
In accordance with one aspect of the present invention a received image in a binary image is warped to counter registration errors predicted or expected to occur upon image output.
In accordance with another aspect of the present invention, a method further includes adjusting the warped image representation to compensate for darkness errors associated with a fractional displacement of an output pixel.
In accordance with one embodiment of the present invention, a registration system for image data in a binary image path includes a registration parameter source which provides registration parameters determined to correct for registration errors, or pixel misplacement during image output. The system also includes a warping processor in data communication with the registration parameter source. The warping processor applies a selected registration parameter to an element of the image data which results in a warped data element.
In accordance with another aspect of the present invention, the system further includes an image output element, such as a light emitting diode bar array, ink jet, ion writing device, or other such device known for image writing, in data communication with the rendering processor which receives the plurality of print ready data elements and outputs a physical representation thereof.
In accordance with another aspect of the present invention, the registration system further includes an image reducing device in data communication with the warping processor where the reducing device converts groups of high addressable image data into a single pixel by an operation such as averaging.
In accordance with another embodiment of the present invention, a method for effectively registering a plurality of individually positioned pixels against a predetermined reference, includes warping a pixel stream to correct registration offset from the reference. Typically, the offset is predicted to occur upon image output. The warped pixel stream is then resampled into a selected output ready form.
In accordance with another aspect of the present invention, the method for registering a plurality of individually positioned pixels further includes modulating a light source through a lens corresponding to the resampled output ready form to expose a surface of a photoreceptor.
In accordance with another aspect of the present invention, the resampling step includes estimating a quantization error induced by the warping step in an individual pixel and distributing that error among yet-to-be-processed pixels by halftoning, error diffusion, pattern substitution and the like.
In accordance with another aspect of the present invention, the method further includes adjusting the warped image representation to compensate for darkness errors associated with a fractional displacement of an output pixel.
One advantage of the present invention resides in the ability to electronically register output images in the binary image path.
Another advantage of the present invention resides in the ability to convert one image of a binary image path at a first resolution to an image of a binary image path at a second resolution.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.